Hyperpolarized xenon (129Xe) is becoming the contrast agent of choice in a broad spectrum of diagnostic protocols. Specifically, hyperpolarized 129Xe offers extraordinary potential as a contrast agent for magnetic resonance imaging (“MRI”).
129Xe is hyperpolarized by spin-exchange optical pumping using gas mixtures of Xe (with natural abundance of 129Xe or enriched in 129Xe), a quenching gas (nitrogen or hydrogen) and a buffer gas. 129Xe comprises only a fraction of the total gas mixture. One embodiment of this method is described in U.S. patent application Ser. No. 09/904,294.
The hyperpolarized 129Xe is then separated from the other gases in the gas mixture and accumulated by freezing at a temperature below the freezing point of 129Xe but above the freezing point of the other gases and in a high strength, e.g., 3,000 Gauss, magnetic field. The frozen hyperpolarized 129Xe has a longer polarization lifetime if it is then kept at temperatures closer to the temperature of liquid nitrogen rather than at temperatures close to its freezing point.
In the prior art, a preferred method of freezing the 129Xe uses a counter flow cold trap—cooled by liquid nitrogen or some other cryogen, as described in U.S. Pat. No. 5,809,801. The gas mixture is flowed down the insulated center cell of two concentric cells immersed in liquid nitrogen and up the outer cell. The frozen 129Xe is deposited at the bottom of the concentric cells where it first contacts the cold surface of the outer cell. As more 129Xe is flowed through the concentric cells, additional frozen 129Xe is deposited on the previously frozen 129Xe at the bottom of the concentric cells, thereby creating a lump of frozen 129Xe.
This method of accumulating hyperpolarized 129Xe has two significant drawbacks. First, the volume of 129Xe that can be accumulated is limited by the available volume at the bottom of the concentric cells. Second, the frozen 129Xe loses polarization faster if its temperature is just below its freezing point as compared to temperatures well below its freezing point. In order to freeze additional gaseous 129Xe through contact with the frozen 129Xe, heat must be transported through the frozen 129Xe, warming it, increasing the relaxation of the frozen 129Xe. In addition, the lump of frozen 129Xe cannot be thawed quickly and dwells near its freezing point when being thawed for use, again increasing the relaxation of the frozen 129Xe.
The present invention comprises a method and apparatus for separating and accumulating 129Xe that is not dependent on a limited volume in which to accumulate the frozen 129Xe. Moreover, the method and apparatus of the present invention does not freeze 129Xe by bringing gaseous 129Xe into contact with previously frozen 129Xe, thereby warming the frozen 129Xe. IT also provides for the more rapid thawing for use of the frozen 129Xe.